1. Field of the Invention
The present invention relates to a magnetic recording device for recording a frequency modulated (FM) luminance signal on a magnetic tape, and more specifically relates to a magnetic recording device wherein the deterioration of the quality of an image due to a turned sideband component is prevented.
2. Description of the Related Art
In a video cassette deck, to prevent resolution from being deteriorated due to the deterioration of S/N of a high frequency component in a luminance signal, an FM luminance signal is generated based upon a luminance signal the level of a high frequency component of which is enhanced beforehand by using a pre-emphasis circuit (first conventional technique).
Also, in another conventional technique proposed in Japanese Utility Model Unexamined Publication No. Sho. 59-25876, the quality of an image is prevented from being deteriorated by providing a variable trap circuit for removing a lower frequency component of an FM luminance signal to respective luminance signal recording processing circuit and luminance signal reproducing processing circuit (second conventional technique).
Also, there is still another conventional technique proposed in Japanese Patent Unexamined Publication No. Hei. 6-303575 and in this technique, a high frequency component extracted from a video signal is inserted during a vertical blanking interval and is recorded. The deterioration of resolution in case the traveling speed of a video tape is made slower than the standard speed is prevented by interpolating the high frequency component inserted during the vertical blanking interval in the decrease of a high frequency component caused in recording or reproduction in a double speed mode or a triple speed mode (third conventional technique).
However, if the first conventional technique is used, the following problem occurs. That is, if a luminance signal is a signal showing an extremely clear vertical-striped image, an FM luminance signal is a signal the degree of frequency modulation of which is high. In other words, the FM luminance signal is a signal including a high-level high-order sideband. Therefore, as the action of a pre-emphasis circuit is also required if a vertical-striped pattern becomes thin, a high-order sideband is generated in a range exceeding a frequency band defined as an FM luminance signal in the FM luminance signal.
Referring to FIG. 4, the above will be described (in FIG. 4, the frequency of a spectrum 80 represents the frequency of a carrier wave of an FM luminance signal). If a signal component the level of which is high of 2.5 MHz for example is included in a luminance signal, a secondary sideband and a tertiary sideband or a higher-order sideband is generated in addition to a primary sideband corresponding to the above signal component in the FM luminance signal. That is, on the side lower than the frequency of the carrier wave 80 of the FM luminance signal, a primary sideband 81, a secondary sideband 82 and a tertiary sideband 83 are generated. Also, on the side higher than the frequency of the carrier wave 80, a primary sideband, a secondary sideband and a tertiary sideband are similarly generated (higher-order sidebands than the tertiary sideband are not shown).
In the meantime, the frequency of the carrier wave 80 is approximately 4.5 MHz in NTSC system. Therefore, the secondary sideband 82 and the tertiary sideband 83 are signals turned up based upon a frequency of 0 Hz. Therefore, the secondary sideband 82 is turned up to be a sideband 92 and the tertiary sideband 83 is turned up to be a sideband 93. The turned sidebands 92 and 93 are signals in the frequency band of the FM luminance signal. As a result, there is a problem that the sidebands 92 and 93 act as a noise component mixed in the FM luminance signal and the quality of an image is deteriorated (a moire phenomenon occurs).
As the above second conventional technique is to prevent an FM luminance signal from being mixed in a low frequency converted chrominance signal, the effect of a sideband generated by the above turn cannot be removed. Also, as the above third conventional technique is to inhibit the decrease of a high frequency component generated when the traveling speed of a video tape is slow, the effect of a sideband generated by the turn cannot be similarly removed.
The present invention was made to solve the above problems and an object of the present invention is to provide a magnetic recording device wherein the deterioration of the quality of an image caused by turning a sideband can be prevented by acquiring an FM luminance signal of a predetermined frequency in frequency conversion after a modified FM luminance signal the frequency of a carrier wave of which is increased is generated and then, a signal component to be a signal in the frequency band of the FM luminance signal when a turn occurs is removed from the generated modified FM luminance signal.
To achieve the above object, according to a first aspect of the present invention, there is provided a magnetic recording device for recording an FM luminance signal the frequency corresponding to a sink chip of which is a frequency fs and the frequency corresponding to a white peak of which is a frequency fw on a magnetic tape, comprising: a frequency modulation circuit for generating a modified FM luminance signal that is an FM luminance signal the frequency of a carrier wave of which is set to a frequency higher than the frequency of a carrier wave of the FM luminance signal when a minimum frequency of the frequency band of the FM luminance signal is a frequency fa and the relative relationship between the frequency corresponding to the sink chip and the frequency corresponding to the white peak of which is equal to the relative relationship between the frequency fs and the frequency fw; a filter circuit for removing a signal component lower than a cut-off frequency of signal components included in the modified FM luminance signal when a frequency lower than a minimum frequency of the frequency band of the modified FM luminance signal by a frequency acquired by doubling the frequency fa is the cut-off frequency; and a frequency conversion circuit for generating the FM luminance signal by converting the frequency of the modified FM luminance signal sent from the filter circuit by using a local oscillated signal of a frequency lower than the frequency of the carrier wave of the modified FM luminance signal.
That is, the filter circuit removes a signal component of a frequency lower than the cut-off frequency. The cut-off frequency is a frequency lower than the minimum frequency of the frequency band of the modified FM luminance signal by a frequency acquired by doubling the frequency fa. Therefore, even if a turn occurs in frequency conversion, a turned signal component becomes a signal outside the band of the FM luminance signal. That is, the FM luminance signal sent from the frequency conversion circuit is a signal which does not include a sideband generated by a turn in its band.
Also, according to a second aspect of the present invention, there is provided a magnetic recording device for recording an FM luminance signal the frequency corresponding to a sink chip of which is a frequency fs and the frequency corresponding to a white peak of which is a frequency fw on a magnetic tape, comprising: a frequency modulation circuit for generating a modified FM luminance signal that is an FM luminance signal the frequency of a carrier wave of which is set to a frequency higher than the frequency of a carrier wave of the FM luminance signal when a minimum frequency of the frequency band of the FM luminance signal is a frequency fa, the difference between the frequency corresponding to the sink chip and the frequency corresponding to the white peak of which is equal to the difference between the frequency fs and the frequency fw and the relationship between the frequency corresponding to the sink chip and the frequency corresponding to the white peak of which is reverse to the relationship between the frequency fs and the frequency fw; a filter circuit for removing a signal component higher than a cut-off frequency of signal components included in the modified FM luminance signal when a frequency higher than a maximum frequency of the frequency band of the modified FM luminance signal by a frequency acquired by doubling the frequency fa is the cut-off frequency; and a frequency conversion circuit for generating the FM luminance signal by converting the frequency of the modified FM luminance signal sent from the filter circuit by using a local oscillated signal of a frequency higher than the frequency of the carrier wave of the modified FM luminance signal.
That is, the filter circuit removes a signal component of a frequency higher than the cut-off frequency. The cut-off frequency is a frequency higher than the maximum frequency of the frequency band of the modified FM luminance signal by a frequency acquired by doubling the frequency fa. Therefore, even if a turn occurs in frequency conversion, a turned signal component becomes a signal outside the band of the FM luminance signal. That is, the FM luminance signal sent from the frequency conversion circuit is a signal which does not include a sideband generated by a turn in its band.